1. Field of the Invention
The invention relates in general to a display panel and fabricating method thereof, and more particularly to a method for transforming amorphous silicon to poly-silicon by selective laser crystallization and the display panel fabricated by using the same.
2. Description of the Related Art
According to the circuit driving method, the organic light emitting diode display panel can be divided into two groups. One is a passive matrix organic light emitting diode display panel (PMOLED display panel), and the other group is an active matrix organic light emitting diode display panel (AMOLED display panel). The thin film transistor (TFT) and the storage capacitor are used in the AMOLED to control the gray scale performance of the organic light emitting device (OLED).
Generally, the PMOLED has the advantages of low production cost and simple technique, but the inefficient drive current results in the low resolution of the display. Also, the brightness of the lighten pixel of the PMOLED cannot be well maintained. Thus, the PMOLED is usually applied to the product less than 5-inch. On the contrary, the brightness of the lighten pixel of the AMOLED can be well maintained since the storage capacitor is disposed in the AMOLED. Thus, it is no need to drive the OLED to emit the light having high brightness, so that the useful life of the AMOLED is longer than that of the PMOLED. Also, the AMOLED can achieve the requirement of high resolution. Moreover, the circuit drive of the AMOLED is more efficient than that of the PMOLED, and the pixels and TFTs of the AMOLED can be integrated on the glass substrate.
The techniques for manufacturing TFT on the glass substrate include the amorphous silicon (a-Si) process and the low temp polysilicon (LTPS) process. The major differences between the a-Si process and the LTPS process are the electrical characteristics of devices and the complexity of processes. The LTPS TFT possesses higher mobility, but the process for fabricating the LTPS TFT is more complicated. Although the a-Si TFT possesses higher mobility, the process for fabricating the a-Si TFT is simple.
Several methods, including the excimer laser annealing (ELA) method, continuous grain silicon (CGS) method, continuous wave (CW) laser method, sequential lateral solidification (SLS) method and metal induced lateral crystallization (MILC) method, have been suggested to turn the amorphous silicon layer into the polysilicon film. The laser beam, such as an excimer laser, a continuous wave (CW) laser or laser beam pulse, can be used for irradiating the amorphous silicon layer. Recently, laser beam pulse is common in use, and the method of lateral crystallization by causing temperature gradient is also popular.
Taking the sequential lateral solidification (SLS) method for example, the use of the optical phase shift masks that have different transparency can cause a lateral temperature gradient in an amorphous silicon layer so as to induce lateral grain growth.
Additionally, the requirements for the regions of a display panel are different. For example, the AMOLED generally comprises a displaying region and a circuit driving region. Whether the current leakage occurs is critical to the displaying region, and the mobility of carriers is a key point of the electrical characteristics of the circuit driving region. Both of the grain size and grain boundary have an effect on the mobility of carriers. Conventionally, the amorphous silicon of the displaying region and the circuit driving region is turned to the polysilicon by an excimer laser beam. However, the polysilicon formed by the excimer laser beam has the irregular grain size and grain boundary, resulting in non-uniform electrical characteristics of the TFTs for driving the displaying region while the operating voltage is applied. Therefore, the image defects such as dark spot defects and dark line defects could be shown on the displaying region during displaying. Typically, the displaying region presents a more uniform image if the polysilicon layer corresponding to the circuit driving region has a larger grain size and more regular grain boundary.
Therefore, it is desirable to form no-defect silicon grains having larger size and more regular boundary, so as to increase throughput and process efficiency, and decrease the production cost.